Optimal Speed Limits: Limits of Optimality Models

Recent estimates of optimal speed limits on public roads in Norway and Sweden are presented. An optimal speed limit is set to minimize the total costs to society of transport. Travel time, vehicle operating costs, road accidents, traffic noise, and air pollution were considered in the determination of optimal speed limits for Norway and Sweden. Most of these effects do not have market prices but they have been valued monetarily, chiefly by means of stated preference surveys. Four perspectives in determining speed limits are compared: (a) the societal perspective, which includes all effects of speed; (b) the road user perspective, which leaves out those effects that can reasonably be assumed to be external from the road user’s point of view—chiefly the environmental impact; (c) the taxpayer perspective, which leaves out all effects whose costs can be regarded as having already been paid through taxes imposed on the use of motor vehicles; and (d) the residential perspective, according to which the effects of speed are assessed from the point of view of residents along a road. These perspectives are found to give different results. In general the road user perspective and the taxpayer perspective result in the highest speed limits, the residential perspective in the lowest. In some cases, in particular for motorways (freeways), the variation in the total costs of travel is found to be very small for speeds in the range of 70 to 110 km/h, almost making the choice of an optimal speed limit in this range the toss of a coin. According to the societal perspective, optimal speed limits are close to current speed limits in Norway, except on rural highways, where a reduction from 80 to 70 km/h would be optimal. In Sweden, optimal speed limits according to societal perspective are lower than current speed limits on rural highways but are higher than current speed limits in urban areas.